The present invention relates to a method for preparing organoclays, including clays purified in the conventional manner. More particularly, the invention relates to methods for the surface modification of clays using polymeric hydrotropes to produce organoclays with improved efficiency and dispersability in nonpolar solvents and polymer systems.
Organoclays with a wide range of surface wetting characteristics have been described in the literature. It is well known that surface treatment can be used to render hydrophilic clay surfaces compatible with solvents of decreasing polarity such as alcohols, ethers, aromatic and aliphatic hydrocarbons, and the like. Conventional hydrophilic organoclays have been prepared by onium ion exchange using polyethyer substituted quaternary ammonium compounds. These organoclays are dispersible in water-based systems and can be used for rheology control in products such as latex paints. Other methods for preparing organoclays displaying surface properties ranging from hydrophilic to hydrophobic have been produced by surface modification of the clay through polymer adsorption rather than onium ion exchange. For example, clay/polymer intercalates have been produced through direct intercalation of clays with either polymer melts, as described in U.S. Pat. No. 5,955,535, or by contacting a clay slurry with a polymer solution followed by drying. These organoclays can be used in forming composites with thermoplastic or thermosetting resins, however they suffer from the drawback that the efficiency of exfoliation can be low due to the potential for cross linking of the clay platelets by the surface-modifying polymer.
Traditionally, hydrophobic organoclays have been prepared through onium ion exchange from a smectite-type clay by surface exchange with high molecular weight quaternary ammonium salts, such as dimethyl dihydrogenated tallow ammonium chloride, dimethyl benzyl hydrogenated tallow ammonium chloride, and methyl benzyl dihydrogenated tallow ammonium chloride. Other onium ions that have been used include phosphonium and sulfonium ions. Another variation described in the literature for making organoclays involves the preparation of a hydrophobic organoclay by onium ion exchange followed by intercalation of a hydrophilic or hydrophobic polymer melt. However, this method of producing organoclays does not directly bind the organic polymers to the clay surface. Consequently, these organoclays cannot be dispersed in a solvent system without loss of the polymer from the clay surface which leads to uncontrolled changes in the surface wetting properties of the organoclay. Additionally, these types of organoclay intercalates do not completely exfoliate in the absence of specific chemical polymerization reactions. This method of producing organoclays is further limited to organoclays that have been surface treated with onium ions having carbon chain lengths equal to or greater than 12. Moreover, the amount of polymer required to modify the surface hydrophilic-lipophilic balance (HLB) value of the clays is typically from 30 to 100 weight percent, or more, relative to the weight of the organoclay, thereby making this approach both costly and inefficient.
In any organoclay application, and especially in the preparation of nanocomposites, obtaining a good dispersion of the clay has always been problematic. Smectite clays have extremely large surface areas and because of their nanoscale, their behavior is dominated by a complex balance of surface chemical forces. It is well known in the patent literature that maximum organoclay dispersion in organic solvents, and hence gelling efficiency, requires the addition of low-molecular-weight polar organic compounds. Various xe2x80x9cpolar activatorsxe2x80x9d as they are called, have been recommended and include low-molecular-weight ketones and alcoholsxe2x80x94with methanol and acetone being preferred. The polar activators are typically combined with small amounts of water and are used at levels ranging from 20 to 60 weight percent relative to the weight of the organoclay. Propylene carbonate has been recommended where the volatility of the activator is a concern. It is believed that the polar organic compounds encourage delamination and dispersion of the organoclay by solvating the high-molecular-weight ammonium ion at the basal surface of the organoclay which in turn affects the inter-platelet associations (i.e., basal spacing) resulting from the van der Waals attractions between surfactant chains and the clay surface. The small amount of water added with the polar activator promotes gellation via bridging between hydrophilic platelet edges. To this end, full rheological effectiveness requires unobstructed access to the hydrogen bonding sites on the clay edges.
The pioneering work in the 1940s showed that increasing chain length of the amine and increasing amine loading leads to more complete coverage of the basal clay surface. This work is discussed in J. W. Jordan, B. J. Hook, and C. M. Finlayson, J. Phys. Colloid Chem. 54, 1196-1208 (1950). For example, approximately 80 percent of the basal surface is covered by amine molecules lying flat at an octadecylamine loading of 100 milliequivalents per 100 g of clay. However, maximum solvation of the hydrocarbon chains of the amine would require the hydrocarbon chain to lift off from the clay surface thereby exposing a hydrophilic, silicate surface. Jordan postulated that the polar organic activators facilitated the solvation of the hydrocarbon chains by simultaneously lifting the hydrocarbon chains on end and shielding the exposed silicate surface.
Self-activating organoclays have also been described and represent an improvement in performance. Self-activation has been achieved through various approaches including manufacturing and compositional modifications. For example, a common approach is to overtreat a clay with a 10 to 25 percent excess of a quaternary amine above the ion exchange capacity of the clay. To maximize the self-activating characteristic, this treatment approach usually requires that amine exchange of the clay be carried out in the presence of low molecular weight polar activators such as alcohols, ketones, ethers, carboxylic acids, carboxylic esters, and amides, as described in U.S. Pat. No. 4,365,030. In a slight variation on his approach, higher molecular weight anionic compounds such as carboxylic acids having low water solubility (e.g., stearic acid) have been used as self-activating agents in conjunction with amine treatment. In this approach, the anionic carboxylic acid forms a water-insoluble complex which attaches to the basal surface of the clay leaving the edge unobstructed.
Analogous approaches have been used to enhance the exfoliation of organoclays during the preparation of a variety of clay/polymer nanocomposites wherein a high molecular weight polar compound is used to activate the organoclay. Examples of activators which also function to compatibilize the organoclay with the polymer matrix include, polyolefin oligomers with telechelic OH groups and maleic anhydride-modified polyolefin oligomers. Oligomeric activators have been used at levels comparable to those of the low molecular weight polar activators. Because of the higher molecular weight of the oligomeric activators, the total organic loading on the organoclay necessary to achieve the desired degree of exfoliation exceeds 70 to 75 weight percent making this approach both expensive and inefficient. In addition, organic solvents are often required to facilitate intercalation of the oligomer which increases cost and manufacturing difficulty. Additionally, the efficiency with which the high molecular weight compatabilizers increase the basal spacing of the organoclay is surprisingly low. For example, telechelic polyolefins reportedly increase the basal spacing of an amine-treated montmorillonite from 33 xc3x85 to only 38 xc3x85 at a mixture ratio of 1:1. These results are reported in U.S. Pat. No. 6,121,361. This small increase in basal spacing suggests that not all of the oligomer becomes intercalated within the organoclay gallery. Because of the polar functional groups employed by this approach, it is not unreasonable to presume that a portion of the oligomer attaches to the edge of the clay and may actually block access to the organoclay galleries.
In summary, under current methods, large quantities of volatile, low molecular weight, polar activators are required to ensure complete exfoliation of organoclays in nonpolar systems. In the formation of clay/polymer nanocomposites, the volatile, low molecular weight, polar activators are undesirable and are replaced by surface active oligomers. However, the amount of oligomeric activator required is 20 to 100 weight percent, or more, relative to the weight of the organoclay making the approach impracticable.
prior art produces organophilic clays that, at least structurally, bear resemblance to the lamellar liquid crystal (LLC) phases found in oil/water/surfactant systems. When an organic solvent is intercalated within the galleries of the organoclay, the alkyl chains of the onium ion lift off from the clay surface producing an LLC structure. In this system, the hydrophilic silicate surface takes the place of the water surface in the oil/water/surfactant system. And just like the LLC phases, the organoclay analogues are highly viscous when the organoclay is fully exfoliated. While organoclays will spontaneously swell in the presence of a suitable organic solvent, the traditional organoclays will not spontaneously disperse into an excess of the organic solvent. In other words, the capacity of the organoclay to solubilize organics is limited. In this regard, the organoclays bear further resemblance to the LLC phases of oil/water/surfactant systems. It is well known from the surfactant literature that LLC""s have a limited capacity to solubilize organics via intercalation of the hydrocarbon region of the LLC. Even in highly swollen LLC phases, where the hydrocarbon chains of the surfactant adopt a fully extended conformation, the terminal groups of the surfactant chains in adjacent surfactant monomolecular layers remain in contact. This is now understood, from the surfactant literature, to be due to entropic effects wherein the surfactant chains exhibit an order parameter that is intermediate between that of a liquid and a solid. Hence, the LLC phase is not compatible with a bulk isotropic liquid hydrocarbon phase. Furthermore, the common assumption expressed in the nanocomposite literature that the interlayer structure of organoclays is disordered and liquid-like is inconsistent with the behavior of LCC structures found in oil/water/surfactant systems.
It is now understood from the surfactant literature that increased organic solubility in oil/water/surfactant systems can be achieved through the use of hydrotropes. The most effective hydrotropes are typically low-molecular-weight organic compounds that disrupt the normal surfactant packing geometry necessary for the formation of the lamellar structure. High-molecular-weight hydrotropes have also been discovered which lead to enhanced organic solubility in LLC phases, again by disrupting the alkyl chain packing within the LLC phase. The high-molecular-weight hydrotropes include the difunctional surfactants produced by Westvaco, of which the dicarboxylic acid; 5-(and 6-) carboxy-4-hexyl-2-cyclohexene-1-yl octanoic acid is an example. While the incorporation of a hydrotrope can increase the solvating capacity of LLC phases, that capacity is not unlimited and the interlayer spacing does not normally exceed the length of the fully extended hydrocarbon chains. This situation is similar to that of the organoclay/hydrocarbon systems.
The most unusual LLC phases are those containing nonionic surfactants of the type n-alkyl polyethylene glycol ether. The lamellar phase of n-dodecyl tetraethylene glycol ether is reported to be capable of solubilizing alkyl hydrocarbons to such an extent that they form a liquid hydrocarbon layer between the layers of surfactant molecules thereby producing a hydrocarbon layer thickness of 60 xc3x85. This exceptional capacity to solubilize hydrocarbons has been shown to be due to the high motional disorder of the surfactant hydrocarbon chains. In other words, the more liquid-like the surfactant chains, the more compatible they become with an isotropic oil layer.
The relevance of LLC phase behavior to the issue of organoclay exfoliation, and hence nanocomposite formation, is two fold. For exfoliation to occur, the interfacial tension between the organoclay and the organic phase (i.e., polymer phase in nanocomposite systems) must be low enough to permit wetting. However, in light of the previous discussion, this condition is not expected to be sufficient to promote exfoliationxe2x80x94a high motional disorder of the surfactant hydrocarbon chains is also required The impact of surfactant order/disorder was in fact partially recognized in the mid 1950s as discussed in J. W. Jordan and F. J. Williams, Kollid Zeitschrift, 137, 40-48 (1954). Specifically, Jordan showed that parallel alignment of the organoclay platelets, which could result from mechanical working of the wet filter cake prior to and during drying, markedly diminished the dispensability of the dried organoclay.
Under prior art methods, large quantities of volatile, low-molecular-weight, polar activators were required to ensure complete exfoliation of organoclays in nonpolar systems. In the formation of clay/polymer nanocomposites, the volatile, low-molecular-weight, polar activators were undesirable and replaced by surface active oligomers. However, the amount of oligomeric activator required was still 20 to 100 weight percent, or more, relative to the weight of the organoclay making the approach impracticable. The present invention overcomes these, and other problems associated with the design and production of organoclays. This invention is based on the inventor""s discovery that the same entropic effects that create a barrier to unlimited swelling in LLC systems also create a barrier to unlimited swelling and spontaneous exfoliation or organoclays in polymer systems and that increased osmotic pressures within organoclay galleries due to extensive swelling aids particle dispersion and exfoliation. More specifically, the invention is based on the discovery that incorporating high-molecular weight hydrotropes into organoclays results in enhanced swelling capabilities in non-polar systems.
Present invention overcomes the problems associated with the design and production of highly dispersible organoclays through the use of polymeric hydrotropes which are capable of producing enhanced swelling capabilities in nonpolar systems at relatively low polymer loadings. The polymeric hydrotropes are comprised of various low-molecular-weight nonionic polymers. More particularly, the present invention provides organoclays which have a wide variety of uses including water treatment applications, as rheological control agents, and in the preparation of nanocomposites. The present method is particularly valuable because it produces a self-activated clay having an expanded basal spacing with only a minor increase in organic loading.
The present method involves adsorbing a sub-monomolecular layer of a polymeric hydrotrope onto the surface of a clay. For the purposes of this invention a clay has sub-monomolecular layer absorbed thereon if the clay surfaces have less than a substantially uniform film of polymeric hydrotrope that is one molecule thick adsorbed thereon. In addition a clay having a sub-monomolecular layer is a clay for which the adsorption capacity of the surface has not been satisfied. For example, adsorption of polyethylene glycol in amounts less than 0.3 grams of polymer per gram of clay would produce a sub-monomolecular coating. In one embodiment of the present invention, the polymeric hydrotrope is adsorbed in an amount from about 1 weight percent to about 15 weight percent, or from about 1 weight percent to about 10 weight percent relative to the weight of the clay. In certain embodiments, the hydrotrope is adsorbed in an amount from between about 2 and about 4 weight percent relative to the weight of the clay. In addition to the polymeric hydrotrope, a cationic HLB modifying surfactant, such as an amine-type surfactant, is also adsorbed onto the clay surface. This may be accomplished by subjecting the clay to cation exchange with a quaternary ammonium salt. Optionally, the properties of the clay surface can be further modified with swelling agents, such as natural and synthetic waxes. Examples of natural waxes include, but are not limited to, paraffin, microcrystalline montan, and vegetable waxes. Examples of synthetic waxes include, but are not limited to, Fisher-Tropsch, polyethylene, polypropylene, polymethylene, chemically modified waxes, and polymerized alpha-olefins. The waxes are used at levels of about 10-30 weight percent relative to the weight of the organoclay. They are used as swelling agents which provide the organoclays in a palletized form, which makes them easier to handle and speeds the rate of clay exfoliation in polyolefins and other polymers. When the organoclays of the present invention are combined with small amounts of waxes, a transparent, extrudable LLC phase is produced. Suitable clays for use in the process include the micas and smectite clays, with exchange capacities of at least 75 milliequivalents per 100 g of clay. Examples of smectite clays include hectorite, montmorillonite, beidelite, stevensite, and saponite. Synthetic micas and smectites are also acceptable.
Briefly, the process used to produce the organoclays of this invention includes the following general steps. Adsorption of the polymeric hydrotrope on the surface of the clay is achieved by dispersing the clay in a suitable solvent, such as water, dispersing and/or dissolving the polymeric hydrotrope in the solvent and allowing the polymer to adsorb on the surface of the dispersed clay. The clay is also subject to ion exchange with a cationic surfactant, which is usually a quaternary amine. Ion exchange either takes place after polymer adsorption has occurred or as polymer adsorption is occurring. In this latter embodiment, the clay is exposed to a solution containing a mixture of the polymeric hydrotrope and the cationic surfactant. The organoclay can then be separated by filtration, washed with water to remove excess salt resulting from the cation exchange, and dried to a desired solvent content. The resulting organoclay may be dispersed into a compatible solvent including desired organic solvents or used in the preperation of nanocomposites.
In an alternative process the organoclays may be produced by a dry method wherein the dry clay is mixed directly with the hydrotrope and the cationic surfactant. This method is suited for less critical applications like water treatment and drilling muds. Any mixer capable of handling high solids can be used to combine the clay and the reagents. Such mixers are well known in the and include, but are not limited to, pug mills and extruders.
The above described embodiments are set forth in the following description and illustrated in the drawings described hereinbelow.